In many cases, barrier layers are needed to prevent the diffusion of one material to an adjacent material during the preparation or use. As an example, an integrated circuit (IC) device requires extremely thin barrier layers to prevent interdiffusion. In particular the use of barrier layers has become more important as copper (Cu) has been used in metallization of VLSI microelectronic devices due to its high conductivity and relatively low resistance to electromigration. The Cu is now deposited typically by a physical deposition method but as film thickness is reduced, chemical deposition techniques will required to achieve nm-scale thick layers that are conformal. Chemical vapor deposition (CVD) processes permit large-scale manufacturing and the conformal seeding of high aspect ratio inter-level vias in high density integrated circuits (bulk filled by ECD), but the Cu is quite mobile in silicon. Cu diffuses and reacts rapidly with silicon (Si) to form compounds, such as Cu3Si, that can destroy shallow junctions and contacts during thermal annealing steps and can result in degraded device performance or failure.
Effective barriers against Cu diffusion are critical because Cu interdiffusion results in an increase in contact and line resistance, a change in barrier height, formation of a leaky PN junction, embrittlement of the contact layer, and destruction of electrical connections to and throughout the chip. Metal nitride barriers can form reliable diffusion barriers and can provide low electrical resistivities relative to their pure metal counterparts. Titanium nitride (TiN) has been used as a diffusion barrier for aluminum, but displays poor performance for Cu metallization due to excessive interdiffusion. Tantalum nitride (TaN) has been a preferred diffusion barrier for Cu metallization. TaN requires a two-step chemical mechanical polishing (CMP) procedure that results in increased dishing of Cu surfaces and can scratch inter-layer dielectrics (ILDs). Dishing of the Cu is caused by a substantially higher CMP etch rate of Cu compared to TaN.
An alternative to TaN, tungsten nitride (WNx) etches by a single CMP process and results in reduced Cu dishing because the CMP etch rates of Cu and WNx are more similar. It has been shown that WNx is an effective diffusion barrier against Cu penetration at temperatures up to about 750° C. As used herein, WNx is a line compound that includes tungsten nitride stoichiometric endpoints of WN2 and W2N. The predominant crystal structure of WN is hexagonal, WN2 is rhombohedral, and W2N is a face centered cubic structure. W2N has the lowest resistivity of the three (50 μΩ-cm bulk resistivity). For example, a tungsten nitride film diffusion barrier between a tungsten plug and an adjoining Cu metallization layer on the surface of the wafer is shown in FIG. 1, where a tungsten plug 14 extends into a silicon substrate 10 with an overlying Cu layer 16 and an intervening diffusion barrier 12. Ever shrinking and increasingly more aggressive feature sizes in ICs require diffusion barriers that are highly conformal and mechanically and thermally stable. Deposition of conformal and continuous barrier layers of tungsten nitride with CVD at relatively low temperatures on high-aspect-ratio structures is not possible with current processes.
Two common methods for deposition of WNx include: physical deposition techniques, such as sputtering, and CVD, involving the reaction of tungsten halides with ammonia. Each of these methods has associated difficulties. Conventional physical vapor deposition technology involves reactive sputtering from a tungsten target in an atmosphere of gaseous nitrogen. Energized particle techniques, particularly sputtering, generally result in poor step coverage. Poor step coverage of the barrier layer can result in areas of excessively thin or missing barrier material in small features that have high aspect ratios. Sputter deposited layers are prone to the generation of high tensile stresses in adjacent layers that can cause interfacial defects.
Chemical vapor deposition processes for forming WNx can involve the reduction of tungsten halides by ammonia. This requires high deposition temperatures (>700° C.) which are incompatible with low dielectric materials and some metallization layers. The reaction byproducts, such as HF or HCl, are extremely corrosive and can rapidly etch other exposed device layers, including Si and SiO2, and can decrease the operating lifetime of the processing equipment used for the deposition. Finally, adduct formation can result due to gas phase nucleation rather than chemical reaction at a wafer surface.
A single-source metal organic precursor molecule can be used for the MOCVD, or other suitable deposition technique, of tungsten nitride thin films. This can permit deposition at lower temperatures and allow a simplified reactor delivery system that avoids the possibility of adduct formation known to occur during the deposition of WNx from WF6 and NH3.
Chiu et al., J. Mater. Res. 1993, 8, 1353-1360 reports the use of a single-source metal-organic precursor, bis(tert-butylimido)bis(tert-butylamido)tungsten (t-BuN)2W(NH-t-Bu)2), to deposit a WN film. This precursor is a tungsten complex that has two imido ligands (═NR) covalently bonded to a tungsten. Deposition is carried out thermally over a temperature range of 450-650° C., with a very slow growth rate of 20-100 Å/min, to form a film of polycrystalline W2N with lattice parameters of 4.14-4.18 Å. The films have relatively high resistivities of 620-7000 μΩ-cm, with marginal step coverage of 50-85% for a 0.40 μm device feature.
McElwee-White et al., U.S. Pat. No. 6,596,888 reports a mono-imido tungsten precursor of the structure: XyL4-yW(NR), for example, Cl4(H3CCN)W(N-i-Pr), as a single-source metal-organic precursor for the deposition of WNx films. MOCVD, at a temperature of 575-600° C. and at about 350 Torr, results in a WNx layer at a growth rate of approximately 940 Å/min with a layer having a bulk resistivity of 164 μΩ-cm. Auger analysis indicated that the film had the composition of 40-45% W, ˜15% N, 5-10% O, and 30-35% C. Deposition at 700° C. and about 760 Torr yielded a WNx film that formed at a growth rate of approximately 3500 Å/min. The film displayed with a bulk resistivity of 1870 μΩ-cm. The film had a composition of 20-35% W, 10-15% N, 5-15% O and 45-52% C. Deposition at a temperature of 575° C. at approximately 350 Torr yielded a WNx film at a growth rate of approximately 900 Å/min, where the film displayed a bulk resistivity of 2016 μΩ-cm. The film had a composition of 49% W, 18% N, 15% C, and 17% O.
More recently other tungsten complexes with nitrogen-bound ligands have been examined as single source precursors, particularly: amides (Becker et al., Appl. Phys. Lett. 2003, 82, 2239-2241, Becker et al., Chem. Mater. 2003, 15, 2969-2976); imidos (Becker et al., Chem. Mater. 2003, 15, 2969-2976, Becker et al., Chem. Mater. 2003, 15, 2969-2976, Won et al., J. Am. Chem. Soc. 2006, 128, 13781-13788, Gwildies et al., Inorg. Chem. 2010, 49, 8487-8494, Bchir et al., J. Am. Chem. Soc. 2005, 127, 7825-7833, Bchir et al., J. Cryst. Growth 2003, 249, 262-274, Bchir et al., J. Organomet. Chem. 2003, 684, 338-350, El-Kadri et al., Dalton Trans. 2006, 1943-1953, Potts et al Dalton Trans. 2008, 5730-5736); amidinate (Gwildies et al., Inorg. Chem. 2010, 49, 8487-8494); guanidinate (Ajmera et al., J. Vac. Sci. Technol., B 2008, 26, 1800-1807, Rische et al., Inorg. Chem. 2006, 45, 269-277, Rische et al., Surf. Coat. Technol. 2007, 201, 9125-9130); and hydrazido (Koller et al., Inorg. Chem. 2008, 47, 4457-4462, McElwee-White et al., Dalton Trans. 2006, 5327-5333). Thermal decomposition of these complexes in the presence of the appropriate reductive co-reactant gas (usually H2 or NH3) typically resulted in WNxCy films virtually free of oxygen and halogen. However, these precursors generally require high deposition temperatures (>350° C.) for the dissociation of C—N and/or N—N bonds that are present in these complexes. High temperatures can be problematic with many low-κ interfacial materials. The decomposition of hydrazido precursors to a WNxCy film at temperatures can occur at about 300° C. Crystallization of amorphous WNxCy films can occur at higher temperature, which can lead to reduced barrier layer performance due to the formation of grain boundaries.
Of the tungsten nitride barriers investigated, stoichiometries ranging from W2N to WN have been observed. Tungsten rich WNx barriers with a stoichiometry close to W2N have proven to be desirable due to a low resistivity and low Cu migration rate. Similarly, WNxCy films that are tungsten rich, approaching W2NC, have resulted in good Cu barriers with low resistivity. However, WNx or WNxCy precursors that can be deposited at temperature less than 300° C. remain a goal for the preparation of diffusion barriers.